1)i don't know for sure what wears out exactly but its usually the rotor and/or the stator, the magnets get weak and wear out
2) you dont have to ugrade rotors its just about tracks and preference mostly i never have and im fine with it cause my motor runs great
3) and yes this is true both under gearing and over gearing can cause heat
4) timing on the esc can be done manually or through a program card or PC if esc allows, on the motor its adjusted from the endbell usually by loosening 3 screws on the back, but remember more timing= more heat from both esc and motor
5) Pretty much all 13.5 motors out there will do u good
i use the dualsky z5 in my cars check em out here ckhobby.com i also run the xc550tf-mj esc and with the usb this esc is a dream
6) the rs pro would not be needed unless you are running motors like 5.5 turn and down but people just like them because of the better fets and such and it does run a little cooler but for 13.5 it wont be needed youll just be paying extra for nothing really
7) sensorless is generally used for rtr's (they usually come with it) and bashing, sensored is the way to go for racing
hope this helps